1. Field of the Invention
This invention relates to an improved cooling insert for casting molds and an associated method and, more specifically, it relates to a highly efficient cooling system which resists undesirably high levels of shrinkage porosity in cast metal articles.
2. Description of the Prior Art
It has long been known to manufacture metal articles by die casting machines wherein a mold cavity defines the shape of the article to be molded and molten metal is introduced into the cavity through runners and gates. It has also been known in connection with such die casting systems to employ circular channels extending through the mold spaced from the mold cavity. A coolant liquid, such as oil or water would circulate in the channels to effect cooling of the molten metal in the desired manner. U.S. Pat. No. 3,667,248 discloses coolant passages within a mold and also discloses the use of a coolant probe.
One of the problems with traditional cooling techniques is that such cooling channels, which may have diameters on the order of about 5/8 inch, have a relatively low heat removal rate as the cooling surface areas are small and convective heat transfer rates are low. Ineffective cooling can result in undesired high levels of shrinkage porosity within the casting which, in turn, results in inferior mechanical properties in the cast article. The problem is particularly acute with respect to castings wherein a thickened wall portion is disposed downstream (with respect to the direction of metal flow) of a thinner wall portion such that the thin portion solidifies long before the thick portion thereby producing excess porosity in the thick wall portion of the casting as it cools. After the casting cavity is completely filled with molten metal, high intensification pressures which may be on the order of about 12,000 psi are applied to the metal to feed metal to the regions of shrinkage porosity. A problem arises when the casting has metal fed from the thin walls which solidify first as the thick wall does not have a direct feed of molten metal to compensate for its shrinkage in volume and, therefore, shrinkage cavities develop in the thick wall. There is a limit to how rapidly the isolated thick wall portions can be caused to freeze through high cooling rates to produce relatively small and distributed porosity in such isolated thick walls. U.S. Pat. No. 4,834,166 discloses the use of a cooling mechanism and heating mechanism within a mold assembly employed to make ceramic molds.
U.S. Pat. No. 4,062,399 discloses the use of a plug member as part of a chill plate employed in combination with a ceramic mold. U.S. Pat. No. 3,903,956 discloses the use of cooling water which passes through an insert block in a die casting machine. Standard round cooling channels are employed in the insert block.
It has been known to employ cooling means in mold inserts which function to define a portion of the mold cavity. See, for example, U.S. Pat. Nos. 4,162,700, 4,637,451, and 4,754,799.
U.S. Pat. No. 4,356,858 discloses die casting machines having water containing mold manifolds which have fingers projecting therefrom to provide water at elevated temperatures so as to preheat the die to operating temperature.
U.S. Pat. No. 4,993,473 discloses the use of generally cylindrical heat conductive chill members in the in-gates.
U.S. Pat. No. 4,899,805 discloses cooling of a die by providing a plurality of curved mold surfaces annularly about the circumference of the exterior of the mold defining member and cooperating curved septum surfaces so as to define curved flow conduits for coolant flow therebetween. Means are provided to facilitate removal of nucleate bubbles and means for breaking up the viscous sublayer of the coolant liquid adjacent to the heat exchange surface. The breaking up of the viscous sublayer is said to be achieved by providing roughening projections ranging in dimension from about 0.3 times the thickness of the viscous sublayer to about twice the thickness of the viscous sublayer and transition zone. It also discloses placing micro-cavities in the heat exchange surface to promote efficient nucleate boiling. The micro-cavities have dimensions on the order of 10.sup.-4 to 10.sup.-2 mm.
In spite of the foregoing disclosures, there remains a very real and substantial need for a die casting mold and associated method which will efficiently cool portions of the mold so as to eliminate excessive porosity in the molded article. This problem is particularly acute with respect to thicker wall portions which are disposed downstream in respect of the direction of metal flow from thinner wall portions which will tend to solidify earlier, thereby obstructing further interdendritic flow of the molten metal to the thicker wall portions during solidification.